Heat-resistant layer for dye-donor element

ABSTRACT

Dye-donor element for use according to thermal dye transfer methods comprising a support having on one side a dye layer and on the other side a heat-resistant layer comprising a binder and inorganic particles having a volume average particle size of at least 1 μm, said heat-resistant layer optionally carrying a topcoat comprising a lubricant, wherein said inorganic particles substantially consist of a mixture of a first type of inorganic particles, which are silicate particles having a Mohs hardness below 2.7, and of a second type of inorganic particles, which are silicate or carbonate particles having a Mohs hardness of at least 2.7 in a ratio by weight of said first type to said second type of inorganic particles comprised between 20:1 and 1:2.

FIELD OF THE INVENTION

The present invention relates to dye-donor elements for use according tothermal dye sublimation transfer and in particular to a heat-resistantlayer of said dye-donor elements.

BACKGROUND OF THE INVENTION

Thermal dye sublimation transfer also called thermal dye diffusiontransfer is a recording method in which a dye-donor element providedwith a dye layer containing sublimable dyes having heat transferabilityis brought into contact with a receiver sheet and selectively, inaccordance with a pattern information signal, is heated by means of athermal printing head provided with a plurality of juxtaposedheat-generating elements or resistors, so that dye is transferred fromthe selectively heated regions of the dye-donor element to the receiversheet and forms a pattern thereon, the shape and density of which is inaccordance with the pattern and intensity of heat applied to thedye-donor element.

A dye-donor element for use according to thermal dye sublimationtransfer usually comprises a very thin support e.g. a polyester support,one side of which has been covered with a dye layer comprising theprinting dyes. Usually, an adhesive or subbing layer is provided betweenthe support and the dye layer.

Owing to the fact that the thin support softens when heated during theprinting operation and then sticks to the thermal printing head, therebycausing malfunction of the printing apparatus and reduction in imagequality, the back of the support (the side opposite to that carrying thedye layer) is typically provided with a heat-resistant layer tofacilitate passage of the dye-donor element past the thermal printinghead. An adhesive layer may be provided between the support and theheat-resistant layer.

The heat-resistant layer generally comprises a lubricant and a binder.In the conventional heat-resistant layers the binder is either a curedbinder as described in e.g. EP 153,880, EP 194,106, EP 314,348, EP329,117, JP 60/151,096, JP 60/229,787, JP 60/229,792, JP 60/229,795, JP62/48,589, JP 62/212,192, JP 62/259,889, JP 01/5884, JP 01/56,587, andJP 02/128,899 or a polymeric thermoplast as described in e.g. EP267,469, JP 58/187,396, JP 63/191,678, JP 63/191,679, JP 01/234,292, andJP 02/70,485).

When multiple prints have to be made using high printing energies in theabsence of any cleaning procedures of the thermal printing head, aresidue resulting from the binder may form on the heat-generatingelements of said thermal printing head and, as a consequence, causemalfunction of the printing device and defects such as jamming,scratching of the printed image, and breakdown of the heat-generatingelements. This phenomenon occurs in particular when the average printingpower of said heat-generating elements exceeds 4.5 W/mm². The averageprinting power is calculated as the total amount of energy appliedduring one line time divided by the line time and by the surface area ofthe heat-generating elements. Conventional thermal printers usuallyoperate with a maximum average printing power of 3 to 4.5 W/mm².However, if higher print densities and/or faster printing speeds arewanted, the average printing power has to be higher than 4.5 W/mm².

These high printing energies are used in thermal sublimation printers,which for the sublimation (or diffusion) of dye require substantiallyhigher printing energies than thermal wax printers, in whichdelamination and fusion of the dye layer are caused.

In case silicone-based lubricants are applied in the form of a separatetopcoat on the heat-resistant layer for improving the slidability of thedye-donor element past the thermal printing head, a higher amount ofdeposited residue is formed unfortunately on said thermal printing head.

It has been suggested in e.g. EP 153,880, EP 194,106, EP 279,467, EP329,117, EP 407,220, and EP 458,538 to incorporate into theheat-resistant layer particles that have a cleaning effect on thethermal printing head during the printing operation. However, softparticles such as organic polymeric beads like e.g. Teflon have nohead-cleaning effect during the printing operation. Silicate particleshaving a Mohs hardness below 2.7 remove dust and loose debris from thesurface during the printing operation, but they have no cleaning effectupon thermally degraded polymer, which actually is left on the thermalprinting head. This phenomenon is observed especially at high printingenergies. Silicate particles having a Mols hardness of at least 2.7remove dust, loose debris, and thermally degraded polymer, but they havea negative effect on the lifetime of the thermal printing head sincethey abrade the passivation layer of said head, especially when they areused at high concentration in the heat-resistant layer.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide adye-donor element for use according to thermal dye transfer methods,said element having favourable slipping properties and neverthelesscausing no substantial contamination of the thermal printing head.

It is also an object of the present invention to provide aheat-resistant layer that minimizes the mechanical wear of thepassivation layer of the thermal printing head so that the lifetime ofthe thermal printing head is enhanced.

Further objects will become apparent from the description hereinafter.

According to the present invention a dye-donor element for use accordingto thermal dye transfer methods is provided, said element comprising asupport having on one side a dye layer and on the other side aheat-resistant layer comprising a binder and inorganic particles havinga volume average particle size of at least 1 μm, said heat-resistantlayer optionally carrying a topcoat comprising a lubricant, wherein saidinorganic particles substantially consist of a mixture of a first typeof inorganic particles, which are silicate particles having a Mohshardness below 2.7, and of a second type of inorganic particles, whichare silicate or carbonate particles having a Mohs hardness of at least2.7 in a ratio by weight of said first type to said second type ofinorganic particles comprised between 20:1 and 1:2.

The present invention further provides a method of forming an image by:

image-wise heating a dye-donor element comprising a support having onone side a dye layer and on the other side a heat-resistant layercomprising a binder and inorganic particles having a volume averageparticle size of at least 1 μm and substantially consisting of a mixtureof a first type of inorganic particles, which are silicate particleshaving a Mohs hardness below 2.7, and of a second type of inorganicparticles, which are silicate or carbonate particles having a Mohshardness of at least 2.7 in a ratio by weight of said first type to saidsecond type of inorganic particles comprised between 20:1 and 1:2, and

causing transfer of the image-wise heated dye to a receiver sheet.

DETAILED DESCRIPTION OF THE INVENTION

The inorganic particles of the first type for use in the heat-resistantlayer according to the present invention, which have a Mohs hardnessbelow 2.7, are salts derived from silica or from silicic acids.

Preferred representatives of inorganic particles of the first type arei.a. clay, China clay, talc, mica, and chlorite.

Especially preferred inorganic particles of the first type are talc,chlorite, and mixtures of both.

Representatives of inorganic particles of the first type are i.a.:

Silicate Typ1.01: Micro Ace Type P3 having a volume average particlesize of 4.5 μm and a Mohs hardness of 1 (commercially available fromNippon Talc, Interorgana Chemiehandel)

Silicate Typ1.02: Mistron Ultramix having a volume average particle sizeof 3.88 μm and a Mohs hardness of 1 (commercially available from CyprusMinerals)

Silicate Typ1.03: Micro-talc I.T. Extra having a volume average particlesize of 4.33 μm and a Mohs hardness of 1 (commercially available fromNorwegian Talc Minerals)

Silicate Typ1.04: Cyprubond (surface-treated to improve adhesion to thebinder) having a volume particle size of 5.28 μm and a Mohs hardness of1 (commercially available from Cyprus Minerals).

Silicate Typ1.05: MP10-52 having a volume particle size of 3.15 μm and aMohs hardness of 1 (commercially available from Pfizer Minerals)

Silicate Typ1.06: MP12-50 having a volume particle size of 2.60 μm and aMohs hardness of 1 (commercially available from Pfizer Minerals)

Silicate Typ1.07: Stellar 600 having a volume average particle size of5.16 μm and a Mohs hardness of 1 (commercially available from CyprusMinerals)

Silicate Typ1.08: Micro Ace Type K1 having a volume average particlesize of 4.75 μm and a Mols hardness of 1 (commercially available fromNippon Talc, Interorgana Chemiehandel)

Silicate Typ1.09: Cyprusperse (chlorite), which is a magnesium aluminiumsilicate having a volume average particle size of 5.57 μm and a Mohshardness of 2 (commercially available from Cyprus Minerals)

Silicate Typ1.10: Iriodin 111, which consists of mica particles having avolume average particle size of 4.42 μm and a Mohs hardness of 2.5(commercially available from Merck)

Silicate Typ1.11: Westmin 8-E, which consists of talc particles having avolume average particle size of 4.41 μm and a Mohs hardness of 1(commercially available from Westmin Talc)

Silicate Typ1.12: Pangel S9, which is an aluminium magnesium silicatehaving a volume average particle size of 4.84 μm and a Mols hardness of2-2.5 (commercially available from Keyser & MacKay)

Silicate Typ1.13: Microline a3, which is a talc having a volume averageparticle size of 2.35 μm and a Mohs hardness of 1 (commerciallyavailable from Talc de Luzenac)

Silicate Typ1.14: Microline a5, which is a talc having a volume averageparticle size of 2.95 μm and a Mohs hardness of 1 (commerciallyavailable from Talc de Luzenac)

Silicate Typ1.15: Microline a7, which is a talc having a volume averageparticle size of 4.09 μm and a Mohs hardness of 1 (commerciallyavailable from Talc de Luzenac)

Silicate Typ1.16: Steamic 005, which is a mixture of talc and chlorite(24% by weight) having a volume average particle size of 2.77 μm and aMohs hardness of 1-2 (commercially available from Talc de Luzenac)

Silicate Typ1.17: Luzenac 10M005, which is a mixture of talc andchlorite (24% by weight) having a volume average particle size of 4.78μm and a Mohs hardness of 1-2 (commercially available from Talc deLuzenac)

Silicate Typ1.18: Luzenac 10M2, which is a mixture of talc and chlorite(50% by weight) having a volume average particle size of 5.19 μm and aMohs hardness of 1-2 (commercially available from Talc de Luzenac)

Suitable inorganic particles of the second type for use in theheat-resistant layer according to the present invention, which have aMohs hardness of at least 2.7, are i.a. amorphous silica, quartz,calcium carbonate, calcium magnesium carbonate (dolomite), and magnesiumcarbonate. Among these amorphous silica particles and calcium magnesiumcarbonate particles are especially preferred.

Representatives of the second type of inorganic particles, which have aMohs hardness of at least 2.7, are i.a.:

Silicate Typ2.01: Syloid 244, which is an amorphous silica having avolume average particle size of 1.96 μm and a Mohs hardness of about 4(commercially available from Grace Davidson)

Silicate Typ2.02: Syloid 266, which is an amorphous silica having avolume average particle size of 2.11 μm and a Mohs hardness of about 4(commercially available from Grace Davidson)

Silicate Typ2.03: Syloid 378, which is an amorphous silica having avolume average particle size of 3.45 μm and a Mohs hardness of about 4(commercially available from Grace Davidson)

Silicate Typ2.04: amorphous monodisperse silica particles prepared byheating Tospearl 120 (polymethylsilylsesquioxan) for 4 h at 700° C. andhaving a volume average particle size of 1.87 μm and a Mohs hardness ofabout 4 (Tospearl 120 is available from Toshiba Silicones)

Silicate Typ2.05: amorphous monodisperse silica particles prepared byheating Tospearl 130 (polymethylsilylsesquioxan) for 4 h at 700° C. andhaving a volume average particle size of 2.57 μm and a Mohs hardness ofabout 4 (Tospearl 130 is available from Toshiba Silicones)

Silicate Typ2.06: amorphous monodisperse silica particles prepared byheating Tospearl 145 (polymethylsilylsesquioxan) for 4 h at 700° C. andhaving a volume average particle size of 3.57 μm and a Mohs hardness ofabout 4 (Tospearl 145 is available from Toshiba Silicones)

Silicate Typ2.07: Min-u-sil 5, which is a crystalline silica (quartz)having a volume average particle size of approximately 5 μm and a Molshardness of 7 (commercially available from Pennsylvania Glass SandCorporation)

Silicate Typ2.08: Sikron C800, which is a crystalline silica (quartz)having a volume average particle size of approximately 5 μm and a Mohshardness of 7 (commercially available from Sifraco, Compiegne)

Dolomite Typ2.09: Microdol Super, which is a calcium magnesium carbonatehaving a volume average particle size of 3.05 μm and a Mohs hardness of3.5 (commercially available from Norwegian Talc)

Dolomite Typ2.10: Microdol Extra, which is a calcium magnesium carbonatehaving a volume average particle size of 4.08 μm and a Mols hardness of3.5 (commercially available from Norwegian Talc)

Dolomite Typ2.11: Myanit 0-10, which is a calcium magnesium carbonatehaving a volume average particle size of approximately 4 μm and a Mohshardness of 3.5 (commercially available from Norwegian Talc)

The mixture of said first type of inorganic particles having a Mohshardness below 2.7 and of said second type of inorganic particles havinga Mohs hardness of at least 2.7 is prepared by simply adding both typesof particles to one another and stirring or agitating them. Theresulting mixture can then be added as such to the coating compositionfor the heat-resistant layer. It is also possible to add both types ofparticles separately to the coating composition for the heat-resistantlayer.

Mixtures of said first type of inorganic particles having a Mohshardness below 2.7 and of said second type of inorganic particles havinga Mohs hardness of at least 2.7 are also commercially available.

A commercially available mixture is e.g.:

Blend 1: Micro-talc AT Extra, which is a mixture of talc and magnesiumcarbonate having a volume average particle size of 4.32 μm and a Mohshardness of 4.32 (commercially available from Norwegian Talc Minerals)

The particle size of the inorganic particles for use in theheat-resistant layer according to the present invention is a volumeaverage particle size as measured by means of a Coulter Multisizer IIhaving an aperture of 30 μm. A particle having a size of 5 μm(Dynosphere SS-051-P) is used to calibrate the apparatus. Thecalibration constant is 349.09. The inorganic particles are dispersed inan aqueous 0.1N sodium chloride solution comprising a fluorinesurfactant before the measurement of the particle size and of theparticle size distribution. The measurement is performed for particlesizes ranging from 0.7 to 22.4 μm. The selected siphon mode is 500 μl.

The volume average particle size of said inorganic particles having aMohs hardness below 2.7 preferably ranges from 3 to 7 μm, whereas thatof said inorganic particles having a Mols hardness of at least 2.7preferably ranges from 1 to 4.5 μm.

At least one kind of inorganic particles having a Mohs hardness below2.7 can be mixed with at least one kind of inorganic particles having aMohs hardness of at least 2.7 for use in the heat-resistant layeraccording to the present invention. Although the ratio by weight ofinorganic particles of said first type to inorganic particles of saidsecond type is normally comprised between 20:1 and 1:2, it is usuallypreferable that the weight of inorganic particles of said first typeexceeds the weight of inorganic particles of said second type, since theinorganic particles of the first type do not at all abrade thepassivation layer of the thermal printing head. A preferred ratio byweight of first type to second type particles is therefore comprisedbetween 10:1 and 3:1.

The total amount of inorganic particles in the heat-resistant layer isgenerally not higher than 1 g/m2 and smaller amounts usually suffice toclean the thermal printing head during the printing operation.Preferably, 5 to 100 mg/m2 of inorganic particles having a Mohs hardnessbelow 2.7 and 2 to 30 mg/m2 of inorganic particles having a Mohshardness of at least 2.7 are used in the heat-resistant layer.

Colloidal silica such as Aerosil R972 (Degussa) can further be added tothe heat-resistant layer according to the present invention. Althoughmixtures of silicate particles having a Mohs hardness below 2.7 withcolloidal silica having a particle size below 1 μm are generally usefulin a heat-resistant layer, it is epecially preferred to add colloidalsilica to the heat-resistant layer according to the present invention.

The binder for the heat-resistant layer can be a cured binder or apolymeric thermoplast.

A cured binder can be produced by a chemical reaction as described ine.g. EP 153,880 and EP 194,106, or by the influence of moisture asdescribed in e.g. EP 528,074, or by irradiation of a radiation-curablecomposition as described in e.g. EP 314,348 and EP 458,538.

Thanks to the fact that the coating procedure of polymeric thermoplastsis very convenient, they are preferably used as binder for theheat-resistant layer. Preferred polymeric thermoplasts are those havinga glass transition temperature above 100° C.; these thermoplasts aresuited for use as binder in the heat-resistant layer, because they aredimensionally stable at higher temperatures. Polymers having a glasstransition temperature above 170° C. are especially preferred. Even morepreferred polymeric thermoplasts are those that are soluble inecologically acceptable solvents such as ketones (e.g. ethyl methylketone and acetone) and alcohols (e.g. isopropanol).

Representatives of polymeric thermoplasts that are suited for use asbinder in the heat-resistant layer are e.g.poly(styrene-co-acrylonitrile), polycarbonates derived from bisphenol A,polyvinyl butyral, polyvinyl acetal, ethyl cellulose, cellulose acetatebutyrate, cellulose acetate propionate, and polyparabanic acid.

Especially preferred polymeric thermoplasts are the polycarbonatesderived from a bis-(hydroxyphenyl)-cycloalkane corresponding to generalformula (I): ##STR1## wherein: R¹, R², R³, and R⁴ (same or different)represent hydrogen,

R¹, R², R³, and R⁴ (same or different) represent hydrogen, halogen, a C₁-C₈ alkyl group, a substituted C₁ -C₈ alkyl group, a C₅ -C₆ cycloalkylgroup, a substituted C₅ -C₆ cycloalkyl group, a C₆ -C₁₀ aryl group, asubstituted C₆ -C₁₀ aryl group, a C₇ -C₁₂ aralkyl group, or asubstituted C₇ -C₁₂ aralkyl group; and

X represents the atoms necessary to complete a 5- to 8-memberedalicyclic ring, which optionally carries at least one C₁ -C₆ alkyl groupor at least one 5- or 6-membered cycloalkyl group, or carries a fused-on5- or 6-membered cycloalkyl group.

These polycarbonates provide a better heat-stability to theheat-resistant layer than conventional polymeric thermoplasts. They alsohave higher glass transition temperatures (Tg), typically in the rangeof about 180° C. to about 260° C., than polycarbonates derived frombisphenol A (Tg of about 150° C.). The polycarbonates can behomopolycarbonates as well as copolycarbonates.

Preferably one to two carbon atoms of the group of atoms represented byX, more preferably only one carbon atom of that group, carry (carries)two C₁ -C₆ alkyl groups on the same carbon atom. A preferred alkyl groupis methyl. Preferably, the carbon atoms of the group of atomsrepresented by X, which stand in α-position to the diphenyl-substitutedcarbon atom, do not carry two C₁ -C₆ alkyl groups. Substitution with twoC₁ -C₆ alkyl groups is preferred on the carbon atom(s) in β-position tothe diphenyl-substituted carbon atom is preferred.

Preferred examples of bis-(hydroxyphenyl)-cycloalkanes corresponding togeneral formula I, which can be employed for preparing thepolycarbonates that can be used according to the present invention arethose comprising 5- or 6-membered alicyclic rings. Examples of suchbis-(hydroxyphenyl)-cycloalkanes are those corresponding to thefollowing structural formulae II to IV. ##STR2##

A particularly preferred bis-(hydroxyphenyl)-cycloalkane is1,1-bis-(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane (formula (II)).

The synthesis of suitable bis-(hydroxyphenyl)-cycloalkanes correspondingto general formula (I) has been described in e.g. DE 3 832 396. Thebis-(hydroxyphenyl)-cycloalkanes are used to prepare high molecularweight thermoplastic aromatic polycarbonates for use according to thepresent invention.

Homopolycarbonates can be prepared from bis-(hydroxyphenyl)-cycloalkanescorresponding to general formula (I), but also copolycarbonates can beprepared by simultaneously using differentbis-(hydroxyphenyl)-cycloalkanes, each of which individually correspondsto the general formula (I).

In the preparation of high molecular weight, thermoplastic, aromaticpolycarbonates for use according to the present invention thebis-(hydroxyphenyl)-cycloalkanes corresponding to general formula (I)can also be used in combination with other hydroxyphenyl compounds thatdo not correspond to general formula (I), e.g. with compounds thatcorrespond to the general formula:

    HO--Z--OH                                                  (VII)

Useful compounds corresponding to general formula (VII) are diphenols,in which Z stands for a bivalent aromatic ring system having from 6 to30 carbon atoms, which ring system contains at least one aromaticnucleus. The aromatic group Z may carry substituents and may containaliphatic or alicyclic residues such as the alicyclic residues containedin the bis-(hydroxyphenyl)-cycloalkanes corresponding to general formula(I) or may contain heteroatoms as bond between the separate aromaticnuclei.

Examples of compounds corresponding to general formula (VII) are i.a.hydroquinone, resorcinol, dihydroxydiphenyl,bis-(hydroxyphenyl)-alkanes, bis-(hydroxyphenyl)-cycloalkanes,bis-(hydroxyphenyl)-sulfide, bis-(hydroxyphenyl)-ether,bis-(hydroxyphenyl)-ketone, bis-(hydroxyphenyl)-sulfone,bis-(hydroxyphenyl)-sulfoxide,α,α'-bis-(hydroxyphenyl)-diisopropylbenzene, and such compounds carryingat least one alkyl and/or halogen substituent on the aromatic nucleus.

These and other suitable compounds corresponding to general formula(VII) have been described in e.g. U.S. Pat. No. 3,028,365, U.S. Pat. No.2,999,835, U.S. Pat. No. 3,148,172, U.S. Pat. No. 3,275,601, U.S. Pat.No. 2,991,273, U.S. Pat. No. 3,271,367, U.S. Pat. No. 3,062,781, U.S.Pat. No. 2,970,131, U.S. Pat. No. 2,999,846, DE 1,570,703, DE 2,063,050,DE 2,063,052, DE 2,211,956, FR 1,561,518, and in "Chemistry and Physicsof Polycarbonates", Interscience Publishers, New York, 1964.

Other preferred compounds corresponding to general formula (VII) arei.a. 4,4'-dihydroxydiphenyl, 2,2-bis-(4-hydroxyphenyl) -propane,2,4-bis-(4-hydroxyphenyl)-2-methylbutane,1,1-bis-(4-hydroxyphenyl)-cyclohexane,α,α'-bis-(4-hydroxyphenyl)-p-diisopropyl-benzene,2,2-bis-(3-methyl-4-hydroxyphenyl)-propane,2,2-bis-(3-chloro-4-hydroxyphenyl)-propane,bis-(3,5-dimethyl-4-hydroxyphenyl)-methane,2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)-propane,bis-(3,5-dimethyl-4-hydroxyphenyl)-sulfone,2,4-bis-(3,5-dimethyl-4-hydroxy-phenyl)-2-methylbutane,1,1-bis-(3,5-dimethyl-4-hydroxyphenyl)-cyclohexane,α,α'-bis-(3,5-dimethyl-4-hydroxyphenyl)-p-diisopropylbenzene,2,2-bis-(3,5-dichloro-4-hydroxyphenyl)-propane, and2,2-bis-(3,5-dibromo-4-hydroxyphenyl)-propane.

Especially preferred compounds corresponding to general formula (VII)are i.a. 2,2-bis-(4-hydroxyphenyl)-propane,2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)-propane,2,2-bis-(3,5-dichloro-4-hydroxyphenyl)-propane,2,2-bis-(3,5-dibromo-4-hydroxyphenyl)-propane, and1,1-bis-(4-hydroxyphenyl)-cyclohexane.

Especially preferred is 2,2-bis-(4-hydroxyphenyl)-propane (bisphenol A).

Incorporation of bisphenol A in the polycarbonate for use according tothe present invention reduces the brittleness of the polycarbonate. Thisresults in less scratches caused by the contaminated thermal printinghead in the transferred image. However, by incorporation of bisphenol Athe glass transition temperature is decreased as compared with that ofthe homopolycarbonate. A compromise has thus to be found betweenscratching and heat-stability.

At least one compound corresponding to general formula (VII) can be usedin combination with bis-(hydroxyphenyl)-cycloalkanes corresponding togeneral formula (I).

If in the preparation of polycarbonates according to the presentinvention the bis-(hydroxyphenyl)-cycloalkanes corresponding to generalformula (I) are used together with at least one compound correspondingto general formula (VII), the amount of bis-(hydroxyphenyl)-cycloalkanescorresponding to general formula (I) in the mixture is at least 10 mol%, preferably at least 25 mol %.

According to another preferred embodiment the polycarbonate for useaccording to the present invention is derived from 100 mol % ofbis-(hydroxyphenyl)-cycloalkanes corresponding to the above generalformula (I).

The high molecular weight polycarbonates can be prepared according topreparation methods for polycarbonates known in the art. Thebis-(hydroxyphenyl)-cycloalkane units and the units resulting from thecompounds corresponding to general formula (VII) can be present in thepolycarbonate in different blocks or the different units can bedistributed randomly.

The isolation of the polycarbonates is performed as known in the art.

The polycarbonates can also be prepared in homogeneous phase accordingto a known method (the so-called pyridine method) or according to theknown melt ester-interchange process by using e.g. diphenyl carbonateinstead of phosgene. In this case as well, the polycarbonates areisolated according to methods known in the art.

Preferably, the molecular weight of the polycarbonates is at least 8000,preferably from 8000 to 200,000 and more preferably from 10,000 to80,000.

Polycarbonates derived from bis-(hydroxyphenyl)-cycloalkanescorresponding to formula (I) are used as binder in the heat-resistantlayer of the dye-donor element according to the present invention in anamount of at least 10% by weight, preferably in an amount from 30 to100% by weight. A mixture of two or more of said polycarbonates can alsobe used in the heat-resistant layer.

Examples of polycarbonates that can be used advantageously in accordancewith the present invention are i.a.:

PC1 Homopolycarbonate having the following structure: ##STR3## wherein nhas a value giving a relative viscosity of 1.295 (measured in a 0.5% byweight solution in dichloromethane)

PC2 Homopolycarbonate having the same structure as PC1 but having arelative viscosity of 2.2

PC3 Copolycarbonate having the following structure: ##STR4## whereinx=55 mol % and y=45 mol %; PC3 has a relative viscosity of 1.295.

The binder of the heat-resistant layer of the dye-donor elementaccording to the present invention may also consist of at least twodifferent mixed binders.

The heat-resistant layer of the dye-donor element according to thepresent invention may in addition to said inorganic particles and thebinder comprise minor amounts of such other agents like surface-activeagents, liquid lubricants, solid lubricants such as waxes e.g.polyethylene wax, polypropylene wax, and amide wax, or mixtures thereof.

The heat-resistant layer according to the present invention may containother additives provided such materials do not inhibit the anti-stickingproperties of the heat-resistant layer and provided that such materialsdo not scratch, erode, contaminate, or otherwise damage the thermalprinting head or harm image quality. Examples of suitable additives havebeen described in EP 389,153.

Suitable surface-active agents for the heat-resistant layer of thedye-donor element according to the present invention are i.a.: alkylphenyl polyalkylene oxides e.g. Antarox CO 630 (GAF), alkyl polyalkyleneoxides e.g. Renex 709 (ICI), and sorbitol esters e.g. Span 85 (ICI) andTween 20 (ICI).

Preferred lubricants for use in the heat-resistant layer of thedye-donor element according to the present invention arepolysiloxan-based lubricants. Among these polyalkylene oxide-modifiedpolydimethylsiloxans such as Byk 320, Byk 307, and Byk 330 (Byk Cera)and Tegoglide 410 (Goldschmidt) are especially preferred. Mixtures ofalkyl-, aryl-, or alkylaryl-modified polyethylene oxide surface-activeagents with polyalkylene oxide-modified polydimethylsiloxans are alsoespecially preferred because they yield prints having a highly uniformdensity. Examples of such mixtures are i.a. Byk 320 with Antarox CO 630, Byk 320 with Antarox CO 850, and Tegoglide 410 with Antarox CO 630.

The heat-resistant layer of the dye-donor element according to thepresent invention is formed preferably by adding the polymericthermoplastic binder or binder mixture, the inorganic particles, andother optional components to a suitable solvent or solvent mixture,dissolving or dispersing the ingredients to form a coating composition,applying said coating composition to a support, which may have beenprovided first with an adhesive or subbing layer, and drying theresulting layer.

The heat-resistant layer of the dye-donor element may be coated on thesupport or printed thereon by a printing technique such as a gravureprocess.

The heat-resistant layer thus formed has a thickness of about 0.1 to 3μm, preferably 0.3 to 1.5 μm.

Although the above-mentioned ingredients of the heat-resistant layer canbe incorporated in one single layer, it is sometimes preferred toincorporate at least part of the additives such as lubricants and/orsurface-active agents in a separate topcoat on top of the heat-resistantlayer. As a result the lubricants and/or surface-active agents are indirect contact with the thermal printing head and thus lead to improvedslipping properties of the the dye-donor element.

A preferred combination of a heat-resistant layer and a topcoat is aheat-resistant layer comprising said inorganic particles and optionallya surface-active agent and a topcoat comprising apolydimethylsiloxan-based lubricant and optionally an alkyl-, aryl-, oralkylaryl-modified polyethylene oxide surface-active agent.

In case the heat-resistant layer is covered with a topcoat, theinorganic particles incorporated into said heat-resistant layer and/orinto a said underlying subbing layer should protrude from the surface ofsaid topcoat.

Preferably a subbing layer is provided between the support and theheat-resistant layer to promote the adhesion between the support and theheat-resistant layer. As subbing layer any of the subbing layers knownin the art for dye-donor elements can be used. Suitable binders that canbe used for the subbing layer can be chosen from the classes ofpolyester resins, polyurethane resins, polyester urethane resins,modified dextrans, modified cellulose, and copolymers comprisingrecurring units such as i.a. vinyl chloride, vinylidene chloride, vinylacetate, acrylonitrile, methacrylate, acrylate, butadiene, and styrene(e.g. poly(vinylidene chloride-co-acrylonitrile). Suitable subbinglayers have been described in e.g. EP 138,483, EP 227,090, EP 564,010,U.S. Pat. No. 4,567,113, U.S. Pat. No. 4,572,860, U.S. Pat. No.4,717,711, U.S. Pat. No. 4,559,273, U.S. Pat. No. 4,695,288, U.S. Pat.No. 4,727,057, U.S. Pat. No. 4,737,486, U.S. Pat. No. 4,965,239, U.S.Pat. No. 4,753,921, U.S. Pat No. 4,895,830, U.S. Pat. No. 4,929,592,U.S. Pat. No. 4,748,150, U.S. Pat. No. 4,965,238, and U.S. Pat. No.4,965,241. Preferably the subbing layer further comprises an aromaticpolyol such as e.g. 1,2-dihydroxybenzene as described in EP 433,496.

Instead of incorporating the inorganic particles for use in accordancewith the present invention in the heat-resistant layer, they can beincorporated at least partially into a said subbing layer between thesupport and said heat-resistant layer.

Any dye can be used in the dye layer of the dye-donor element of thepresent invention provided it is transferable to the receiver sheet bythe action of heat. Examples of suitable dyes have been described ine.g. EP 432,829, EP 400,706, EP 485,665, EP 498,083, EP 453,020, and inthe references mentioned therein.

The amount ratio of dye or dye mixture to binder generally ranges from9:1 and 1:3 by weight, preferably from 3:1 and 1:2 by weight.

The following polymers can be used as polymeric binder: cellulosederivatives, such as ethyl cellulose, hydroxyethyl cellulose,ethylhydroxy cellulose, ethylhydroxyethyl cellulose, hydroxypropylcellulose, methyl cellulose, cellulose nitrate, cellulose acetateformate, cellulose acetate hydrogen phthalate, cellulose acetate,cellulose acetate propionate, cellulose acetate butyrate, celluloseacetate pentanoate, cellulose acetate benzoate, cellulose triacetate;vinyl-type resins and derivatives, such as polyvinyl alcohol, polyvinylacetate, polyvinyl butyral, copolyvinyl butyral-vinyl acetal-vinylalcohol, polyvinyl pyrrolidone, polyvinyl acetoacetal, polyacrylamide;polymers and copolymers derived from acrylates and acrylate derivatives,such as polyacrylic acid, polymethyl methacrylate and styrene-acrylatecopolymers; polyester resins; polycarbonates;copoly(styrene/acrylonitrile); polysulfones; polyphenylene oxide;organosilicones, such as polysiloxans; epoxy resins and natural resins,such as gum arabic. Preferably, the binder for the dye layer of thepresent invention comprises copoly(styrene/acrylonitrile).

The dye layer may also contain other additives such as i.a. thermalsolvents, stabilizers, curing agents, preservatives, organic orinorganic fine particles, dispersing agents, antistatic agents,defoaming agents, and viscosity-controlling agents, these and otheringredients being described more fully in EP 133,011, EP 133,012, EP111,004, and EP 279,467.

Addition of beads of polyolefin waxes or amid waxes, and/or ofpolymethylsilylsesquioxan particles, as described in EP 554,583, to thedye layer, said beads and/or particles protruding from the surface ofsaid layer, is especially preferred.

Any material can be used as the support for the dye-donor elementprovided it is dimensionally stable and capable of withstanding thetemperatures involved up to 400° C. over a period of up to 20 msec, andis yet thin enough to transmit heat applied on one side through to thedye on the other side to effect transfer to the receiver sheet withinsuch short periods, typically from 1 to 10 msec. Such materials includepolyesters such as polyethylene terephthalate, polyamides,polyacrylates, polycarbonates, cellulose esters, fluorinated polymers,polyethers, polyacetals, polyolefins, polyimides, glassine paper andcondenser paper. Preference is given to a support comprisingpolyethylene terephthalate. In general, the support has a thickness of 2to 30 μm. The support may also be coated with an adhesive of subbinglayer, if desired. Examples of suitable subbing layers have beendescribed in e.g. EP 433,496, EP 311,841, EP 268,179, U.S. Pat. No.4,727,057, and U.S. Pat. No. 4,695,288.

A dye-barrier layer comprising a hydrophilic polymer may also beemployed between the support and the dye layer of the dye-donor elementto enhance the dye transfer densities by preventing wrong-way transferof dye backwards to the support. The dye barrier layer may contain anyhydrophilic material that is useful for the intended purpose. Ingeneral, good results have been obtained with gelatin, polyacrylamide,polyisopropylacrylamide, butyl methacrylate-grafted gelatin, ethylmethacrylate-grafted gelatin, ethyl acrylate-grafted gelatin, cellulosemonoacetate, methyl cellulose, polyvinyl alcohol, polyethyleneimine,polyacrylic acid, a mixture of polyvinyl alcohol and polyvinyl acetate,a mixture of polyvinyl alcohol and polyacrylic acid or a mixture ofcellulose monoacetate and polyacrylic acid. Suitable dye barrier layershave been described in e.g. EP 227,091 and EP 228,065. Certainhydrophilic polymers e.g. those described in EP 227,091 also have anadequate adhesion to the support and the dye layer so that the need fora separate adhesive or subbing layer is avoided. These particularhydrophilic polymers used in a single layer in the dye-donor elementthus perform a dual function, hence are referred to asdye-barrier/subbing layers.

The support for the receiver sheet that is used with the dye-donorelement may be a transparent film of e.g. polyethylene terephthalate, apolyether sulfone, a polyimide, a cellulose ester, or a polyvinylalcohol-co-acetal. The support may also be a reflective one such as abaryta-coated paper, polyethylene-coated paper or white polyester i.e.white-pigmented polyester. Blue-coloured polyethylene terephthalate filmcan also be used as support.

To avoid poor adsorption of the transferred dye to the support of thereceiver sheet this support must be coated with a special layer calleddye-image-receiving layer, into which the dye can diffuse more readily.The dye-image-receiving layer may comprise e.g. a polycarbonate, apolyurethane, a polyester, a polyamide, polyvinyl chloride,polystyrene-co-arcylonitrile, polycaprolactone, or mixtures thereof. Thedye-image receiving layer may also comprise a heat-cured product ofpoly(vinyl chloride/co-vinyl acetate/co-vinyl alcohol) andpolyisocyanate. Suitable dye-image-receiving layers have been describedin e.g. EP 133,011, EP 133,012, EP 144,247, EP 227,094, and EP 228,066.

In order to improve the light resistance and other stabilities ofrecorded images, UV absorbers, singlet oxygen quenchers such asHALS-compounds (Hindered Amine Light Stabilizers) and/or antioxidantsmay be incorporated into the dye-image-receiving layer.

The dye layer of the dye-donor element or the dye-image-receiving layerof the receiver sheet may also contain a releasing agent that aids inseparating the dye-donor element from the receiver sheet after transfer.The releasing agents can also be applied in a separate layer on at leastpart of the dye layer or of the dye-image-receiving layer. Suitablereleasing agents are solid waxes, fluorine- or phosphate-containingsurfactants and silicone oils. Suitable releasing agents have beendescribed in e.g. EP 133,012, JP 85/19,138, and EP 227,092.

The dye-donor elements according to the invention are used to form a dyetransfer image, which process comprises placing the dye layer of thedye-donor element in face-to-face relation with the dye-image-receivinglayer of the receiver sheet and image-wise heating from the back of thedye-donor element. The transfer of the dye is accomplished by heatingfor about several milliseconds at a temperature of 400° C.

Preferably, the average printing power applied by means of a thermalprinting head during the image-wise heating of the dye-donor element ishigher than 4.5 W/mm².

When the image-wise heating process is performed for but one singlecolour, a monochromic dye transfer image is obtained. A multicolourimage can be obtained by using a dye-donor element containing three ormore primary colour dyes and sequentially performing the process stepsdescribed above for each colour. The above sandwich of dye-donor elementand receiver sheet is formed on three occasions during the time whenheat is applied by the thermal printing head. After the first dye hasbeen transferred, the elements are peeled apart. A second dye-donorelement (or another area of the dye-donor element with a different dyearea) is then brought in register with the dye-receiving element and theprocess is repeated. The third colour and optionally further colours areobtained in the same manner.

The following example illustrates the invention in more detail without,however, limiting the scope thereof.

EXAMPLE

A series of dye-donor elements for use according to thermal dyesublimation transfer were prepared as follows.

Polyethylene terephthalate film having a thickness of 6 μm was providedon both sides with a subbing layer from a solution of copolyestercomprising isophthalic acid units/terephthalic acid units/ethyleneglycol units/neopentyl glycol units/adipic acid units/glycerol units inethyl methyl ketone.

A solution comprising 8% by weight of dye A, 4% by weight of dye B, and10% by weight of poly(styrene-co-acrylonitrile) as binder in ethylmethyl ketone as solvent was prepared: ##STR5##

From the resulting solution a layer having a wet thickness of 10 μm wascoated on the subbed polyethylene terephthalate film. The resulting dyelayer was dried by evaporation of the solvent.

A heat-resistant layer having a wet thickness of 10 μm was coated on thesubbed back of the polyethylene terephthalate film from a solution inethyl methyl ketone containing a polycarbonate binder PC1 (13% byweight) and inorganic particles (the nature and amount of which areindicated below in Table 1).

The side of the donor elements that showed the heat-resistant layer wascoated with a solution forming a topcoat, said solution being a 0.5 % byweight solution of Tegoglide 410 (commercially available fromGoldschmidt) in isopropanol.

Receiver sheets were prepared by coating a polyethylene terephthalatefilm support having a thickness of 175 μm with a dye-image-receivinglayer from a solution in ethyl methyl ketone of 3,6 g/m² of poly(vinylchloride/co-vinyl acetate/co-vinyl alcohol) (Vinylite VAGD supplied byUnion Carbide), 0,336 g/m² of diisocyanate (Desmodur VL supplied byBayer AG), and 0,2 gm² of hydroxy-modified polydimethylsiloxan (TegomerH SI 2111 supplied by Goldschmidt).

Each dye-donor element was printed in combination with a receiver sheetin a printer set-up using a Kyocera thermal printing head, TypeKGT-219-12MP4-75PM at an average power of 60 mW per dot (total amount ofenergy applied to one resistor element divided by the total line time,80 mW with a duty cycle of 75%). The surface of the heater elementmeasured 68 by 152 mm. Consequently, the average printing power appliedto the heater elements was 5.8 W/mm2. The printing was repeated 100times for each dye-donor element. All heat-resistant layers asidentified in Table 1 hereinafter allowed easy continuous transportacross the thermal printing head.

Next, the thermal printing head was disconnected from the printer andinspected under an optical microscope (Leitz microscope: enlargement100×) to trace any contamination of the resistors of the thermalprinting head. The following levels of contamination were attributable:excellent (no contamination at all), good (hardly perceptiblecontamination), moderate (clearly visible contamination), and bad(extensive contamination all over the electrode surfaces).

In Table 1 hereinafter (E) stands for excellent, (G) for good, (M) formoderate, and (B) for bad. The amounts of the inorganic particles andbinder are indicated in % by weight calculated on the total weight ofthe coating solution (solvent was added up to 100%). The resultsobtained are listed in Table 1.

                  TABLE 1                                                         ______________________________________                                               Heat-resistant layer                                                          composition       Contamination                                               Particles of                                                                          %      Particles of                                                                             %    (after 100                                     first type                                                                            (wt)   second type                                                                              (wt) prints)                                 ______________________________________                                        Comparison                                                                    Comp.1   --        --     --       --    B*                                   Comp.2   Typ1.01   0.5    --       --   B                                     Comp.3   Typ1.01   1.5    --       --   M                                     Comp.4   Typ1.03   0.5    --       --   B                                     Comp.5   Typ1.04   0.5    --       --   M                                     Comp.6   Typ1.08   0.5    --       --   B                                     Comp.7   Typ1.09   0.5    --       --   M                                     Comp.8   Typ1.01   0.5    Aerosil R972                                                                           5    B                                     Comp.9   --        --     Typ2.01  0.1  B                                     Comp.10  --        --     Typ2.01   0.25                                                                              M                                     Invention                                                                     Inv.1    Typ1.01   0.5    Typ2.01  0.2  E                                     Inv.2    Typ1.01   0.5    Typ2.02  0.1  G                                     Inv.3    Typ1.01   0.5    Typ2.09  0.2  G                                     Inv.4    Typ1.01   0.5    Typ2.09  0.3  E                                     Inv.5    Blend 1   0.5    --       --   G                                     Inv.6    Blend 1   1.0    --       --   E                                     ______________________________________                                         B* means that the thermal printing head was covered entirely with             contaminating deposits.                                                  

Aerosil R972 is a colloidal silica available from Degussa and having aparticle size of 16 nm

The above results show that the best results with respect to avoidingcontamination of the thermal printing head are obtained by the combineduse of inorganic particles having a Mohs hardness below 2.7 andinorganic particles having a Mohs hardness of at least 2.7 in theheat-resistant layer.

It is also clear that the amount of particles having a Mohs hardness ofat least 2.7 can be reduced if these particles are combined withinorganic particles having a Mohs hardness below 2.7. This offers theadvantage that the mechanical wear of the passivation layer of thethermal printing head is minimized so that the lifetime of the latter isenhanced.

We claim:
 1. Dye-donor element for use according to thermal dye transfermethods, said element comprising a support having on one side a dyelayer and on the other side a heat-resistant layer comprising a binderand inorganic particles having a volume average particle size of atleast 1 μm, wherein said inorganic particles substantially consist of amixture of a first type of inorganic particles, that are silicateparticles having a Mols hardness below 2.7, and of a second type ofinorganic particles, that are silicate or carbonate particles having aMohs hardness of at least 2.7 in a ratio by weight of said first type tosaid second type of inorganic particles comprised between 20:1 and 1:2.2. A dye-donor element according to claim 1, wherein the volume averageparticle size of said silicate particles having a Mohs hardness below2.7 ranges from 3 to 7 μm and that of said silicate or carbonateparticles having a Mohs hardness of at least 2.7 ranges from 1 to 4.5μm.
 3. A dye-donor element according to claim 1 or 2, wherein saidparticles having a Mohs hardness below 2.7 are clay, China clay, talc,mica, or chlorite particles.
 4. A dye-donor element according to claim1, wherein said particles having a Mohs hardness of at least 2.7 areamorphous silica particles or calcium magnesium carbonate particles. 5.A dye-donor element according to claim 1, wherein 5 to 100 mg/m2 of saidparticles having a Mohs hardness below 2.7 and 2 to 30 mg/m2 ofparticles having a Mohs hardness of at least 2.7 are present in saidheat-resistant layer.
 6. A dye-donor element according to claim 1,wherein said binder is a polymeric thermoplast.
 7. A dye-donor elementaccording to claim 6, wherein said binder comprises a polycarbonatederived from a bis-(hydroxyphenyl)-cycloalkane corresponding to generalformula (I): ##STR6## wherein: R¹, R², R³, and R⁴ (same or different)represent hydrogen, halogen, a C₁ -C₈ alkyl group, a substituted C₁ -C₈alkyl group, a C₅ -C₆ cycloalkyl group, a substituted C₅ -C₆ cycloalkylgroup, a C₆ -C₁₀ aryl group, a substituted C₆ -C₁₀ aryl group, a C₇ -C₁₂aralkyl group, or a substituted C₇ -C₁₂ aralkyl group; andX representsthe atoms necessary to complete a 5- to 8-membered alicyclic ring, whichoptionally carries at least one C₁ -C₆ alkyl group or at least one 5- or6-membered cycloalkyl group, or carries a fused-on 5- or 6-memberedcycloalkyl group.
 8. A dye-donor element according to claim 1, whereinsaid heat-resistant layer carries a topcoat comprising a lubricant.
 9. Adye-donor element according to claim 8, wherein said lubricant is apolydimethylsiloxane based lubricant.
 10. Method of forming an imageby:image-wise heating a dye-donor element comprising a support having onone side a dye layer and on the other side a heat-resistant layercomprising a binder and inorganic particles having a volume averageparticle size of at least 1 μm and substantially consisting of a mixtureof a first type of inorganic particles, that are silicate particleshaving a Mohs hardness below 2.7, and of a second type of inorganicparticles, that are silicate or carbonate particles having a Mohshardness of at least 2.7 in a ratio by weight of said first type to saidsecond type of inorganic particles comprised between 20:1 and 1:2, andcausing transfer of the image-wise heated dye to a receiver sheet.
 11. Amethod according to claim 10, wherein the average printing power appliedby means of a thermal printing head during said image-wise heating ishigher than 4.5 W/mm².
 12. A method according to claim 10 or 11, whereinsaid heat-resistant layer carries a topcoat comprising a lubricant.